This invention is concerned with techniques for determining the difference between two optical images.
Real-time image processing is becoming an area of great interest in fields, such as robotics, which require the recognition and tracking of objects. A clear advantage to the optical approach for these applications is the capability of parallel processing, with its concomitant increase in processing speed over digital computing techniques. Additional applications for optical image processing include industrial quality assurance, optical logic gates, and the detection of motion in a scene.
In principle, coherent image subtraction and addition can be achieved with an interferometer, such as the Mach-Zender or Michelson configurations. Image subtraction is accomplished with an interferometer by destructive interference, which requires that the phases of the illuminating beams be 180.degree. out of phase throughout the two dimensional region within which the images overlap. In practice, however, the intensity of the overlapped image in an interferometer tends to drift between its maximum and its minimum. This error is caused by phase fluctuations due to ambient air currents and thermal drift. Another disadvantage of the conventional interferometric approach arises from the exact alignment which is required, because such interferometers are extremely sensitive to misalignment.
A phase-conjugate interferometric image subtracter has been developed to alleviate these problems with the conventional interferometric approach (See Chiou and Yeh, Phase-Conjugate Interferometric Coherent Image Subtraction, U.S. Pat. No. 4,718,749). This coherent image subtracter is self aligning, independent of the optical path length of the interferometer arms (as long as the difference between the two optical path lengths is within the coherence length of the laser light), and is self-corrected for slow phase fluctuations due to air currents or thermal drift. In spite of these advantages, however, there are some limitations to this design. Because this image subtracter relies on self-pumped phase conjugation, the two input beams must compete for the gain needed to initiate and stabilize the self-pumped phase conjugation process. Consequently, the process tends to be slow and unstable. As the self-pumped phase conjugate reflectivity depends not only on the input beam intensity but also on the direction and position of the input beam relative to the photorefractive crystal, a very delicate balancing of the two input beam intensities is also required to achieve equal phase conjugate reflectivities for the two arms. This balancing can be particularly difficult when the input beams are encoded with complicated image information.
Therefore, a need has developed in the art for an improved coherent interferometric image subtraction scheme which can initiate and stabilize the self-pumped phase conjugation process with a low input intensity, minimal time delay, and over a wide dynamic range of image intensities.